1. Field of the Invention
This invention relates to a sheet feeding device provided in various machines provided with a mechanism for feeding sheets, such as a calculator, a copying apparatus, a facsimile apparatus, a word processor and a typewriter, and particularly to a sheet feeding device utilizing a travelling vibration wave.
2. Related Background Art
A device of this kind has heretofore been designed such that as disclosed in Japanese Laid-Open Patent Application No. 59-177243, a travelling wave is formed in resilient members for nipping a sheet therebetween, whereby the sheet is fed.
The principle of sheet conveyance in the above-mentioned proposal will hereinafter be described with reference to FIG. 7 of the accompanying drawings.
A sheet 13 is nipped with a moderate pressure force by and between resilient members 11 and 12. Travelling flexural vibration (travelling wave) is formed in each of the resilient members 11 and 12, and the phase difference between these travelling waves are designed to be spatially 180.degree. and therefore, the flexural vibrations of the respective resilient members 11 and 12 travel so that the convex portions thereof may always be opposed to the sheet 13 side. At this time, material points at which for example, the convex portions of the resilient members 11 and 12 lie generally make a motion describing an elliptical orbit. In FIG. 7, with regard to the resilient member 11, when the travelling wave travels to the right, the material point describes a clockwise elliptical orbit as shown. Thus, the direction of motion of the material points on the convex portions of the resilient members 11 and 12 are opposite to the direction of travel of the vibrations, and this works as a force which transports the sheet 13.
On the other hand, in the concave portions of the resilient members, there is created a sheet moving force in the same direction as the direction of travel, but this pressure force is small as compared with that in the convex portions and therefore, the frictional forces between the sheet 13 and the resilient members 11 and 12 are small and the sheet transporting forces are also small and thus, the sum total of the sheet transporting forces works in the direction opposite to the direction of travel of the aforedescribed flexural vibrations.
As a sheet feeding device using such a principle of sheet conveyance, there is one as shown in FIGS. 5 and 6 of the accompanying drawings. This sheet feeding device uses a pair of annular vibration members 11 and 12 of a track-like planar shape as vibration members, and electrostrictive elements 111 and 121 such as piezo-electric elements are fixed to those surfaces of the annular vibration members which are not opposed to each other. These electrostrictive elements 111 and 121 each are divided into two groups, and an alternating voltage which is 90.degree. out of phase with respect to a voltage applied to one of the two groups may be applied to the other group. When the two AC voltages differing in phase from each other are applied to the two groups of each of the electrostrictive elements 111 and 121, travelling flexural vibrations travelling along the circumferential direction are created in the annular vibration members 11 and 12 by the aforedescribed theory and the sheet 13 is moved by being subjected to forces in the direction opposite to the direction of travel of the travelling flexural vibrations from the annular vibration members. The sheet feeding device using the track-shaped annular vibration members utilizes as a sheet conveying force a travelling flexural motion created in the straight portion of each vibration member, and in this sheet feeding device, the vibration members are disposed in such a manner that the straight portions thereof are equal to the direction of feeding of the sheet. In this sheet feeding device, if the two straight portions of the annular vibration members 11 and 12 are both in contact with the sheet 13, the sheet 13 will no longer move by being subjected to forces in opposite directions at one time and therefore, in this device, the design is made such that only the straight portions 11a and 12a of the vibration members 11 and 12 contact the sheet 13 and the other portions, including the straight portions 11b and 12b of the vibration members, are thin-walled so as not to contact with the sheet 13.
The upper vibration member 11 is mounted on the tip end portion of an arm member 22 having a pin 22a supported by a bearing member 20a provided on the opposite side of a bottom plate 20, and imparts a pressure force to the sheet 13 by the gravity thereof.
Reference numeral 41 designates a sheet stand, and the reference numeral 21 denotes a sheet guide.
Also, the resilient members 11 and 12 are such that the vibration amplitude of the travelling wave differs between the inner side and the outer side of the resilient members and therefore, there is generated torsional vibration. Also, the amplitude becomes greater toward the outer side and therefore, elliptical motion of the material points occurs so that an inwardly directed force (f.sub.2 in FIG. 6) may be created on the surfaces of the resilient members by the combination of the travelling vibration wave and the torsional vibration. Due to this torsional vibration, the locus of the material point on the surfaces of the resilient members is an ellipse about said shaft, and a moving force is created for the sheet in the tangential direction of this ellipse.
As a result, the sheet is fed substantially without inclination and lateral shift during the conveyance thereof.
Now, the sheet feeding device as described above is provided with the sheet guide 21 for preventing inclination and lateral shift, but has suffered from the disadvantage that actually, oblique movement of the sheet cannot be prevented by only the pressure force against the guides 21 like the force f.sub.2 indicated in FIG. 6 which is created on the surface of the resilient member. For example, if in FIG. 6, an extraneous force F.sub.1 is imparted to the sheet 13, the sheet 13 will be inevitably inclined (obliquely moved).